Field of the Invention
This invention relates to the manufacture of flat glass by forming on a bath of molten tin. More particularly, this invention relates to a method of minimizing top surface distortion associated with flat glass produced by forming on a bath of molten tin in which the glass has a relatively short residence time on the molten tin.
Brief Description of the Prior Art
Manufacturing of flat glass by forming on a bath of molten tin has been known since the 1900's and has been described in U.S. Pat. Nos. 710,357 to Heal, 789,911 to Hitchcock, and in U.S. Pat. Nos. 2,991,159; 3,083,551 and 3,220,816, all to Pilkington. Briefly, the process involves delivering a molten glass to a pool of molten tin confined within a forming chamber. Shortly after the initial delivery, the applied tractive forces in withdrawing the glass from the forming chamber attenuate the glass while it is cooling to form a continuous sheet of a ribbon of glass of a desired, predetermined, final shape and thickness which is continuously advanced over the bath of molten tin. To successfully form flat glass on a pool of molten tin, the forming chamber should be maintained as free as possible from oxygen because as the oxygen content of the tin rises, the glass absorbs increasing amounts of stannous tin particularly at the beginning of the forming chamber where the molten glass is discharged onto the molten tin. Subsequent heat treatment such as is used in a bending operation to make curved windshields or in thermal tempering brings about oxidation of the stannous tin to stannic tin resulting in distortion in the bottom or tin-contacting surface of the glass. This distortion is a microscopic wrinkling of the surface which scatters transmitted light and is familiarly referred to in the art as bloom. See FIG. 2 which is a photomicrograph of a bloomed glass surface.
Besides bloom which is on the bottom surface, another optical distortion associated with flat glass formed on a pool of molten tin is top surface distortion familiarly referred to as batter and broken line distortion. This distortion is much grosser than bottom surface bloom and is evidenced not as much by reduction in transmitted light, but is detected when the glass is observed at a small angle by reflected light. Top surface distortion known as batter is shown in the photograph in FIG. 1. Top surface batter seems to develop in the glass early in the forming operation in the hotter regions of the forming chamber when the glass is being attenuated into sheet form before it has attained its final shape and thickness.
Top surface distortion is worse in situations in which the glass has relatively short residence times on the molten tin. Such situations occur, for example, where the length of the forming chamber is relatively short, that is, where the length is less than about six times its width. Conventional forming chambers have length-to-width ratios of about 12 to 16:1. U.S. Pat. Application Ser. No. 483,508, filed June 27, 1974, to Kunkle discloses forming processes in relatively short forming chambers. Also, short residence time situations occur in forming chambers of conventional length (i.e, about 160 feet) for the production of relatively thin glass in which the continuously formed sheet of glass is advanced through the forming chamber at relatively high rates. In both cases, that is, in forming in shorter chambers and in forming thinner glass at relatively high rates in conventionally sized chambers, the residence time of the glass on the tin is relatively short and during this short period of time attenuating forces and cooling rates are quite high which is believed to aggravate top surface distortion.
Until the discoveries of the present invention, the prior workers in the art of glassmaking have not believed that the problems of bottom and top surface distortion were interrelated. The prior art believed that the problems had different origins and had to be dealt with separately. For example, since the origin of bottom surface bloom is stannous tin penetration of the bottom surface of the ribbon, the prior art has recommended various procedures to reduce the oxygen content of the tin bath since it is oxygen which oxidizes the tin metal to stannous tin. Thus, it has been proposed to tightly seal the float forming chamber and to conduct the float forming operation under a reducing atmosphere, for example, 95 percent nitrogen and 5 percent hydrogen gases.
With top surface distortion, the prior art quite reasonably believed that the origin of the problem resided in the top surface of the ribbon. Certain float glass manufacturers in the prior art believed that top surface distortion was due to volatilization of one or more of the volatile constituents of the glass composition from the surface of the glass. This volatilization, which created compositional differences between the ribbon surface and the interior portions of the ribbon, resulted in surface distortion due to different coefficients of thermal expansion between the ribbon surface and the interior as the glass is attenuated and cooled advancing through the tin bath. To control the problem, certain float glass manufacturers recommended altering basic float glass compositions so as to reduce their volatility.